The transition from miaskitic to agpaitic magmas – case study Serra de Monchique, Portugal

The peralkaline F-rich igneous pluton of the Serra de Monchique complex in southern Portugal shows various REE- and HFSE-bearing mineral assemblages in variously evolved and hydrothermally overprinted rock units. Based on mineral compositions and field relations, the differentiation history of the complex will be elucidated with special focus on partitioning of halogens, REEs and HFSEs.

The complex consists of two main units: an inner core unit of homogeneous nepheline to sodalite syenite and an outer roof unit of heterogeneous nepheline and sodalite to quartz-(monzo)syenites and breccias. Faults separate the two main units. The complex is crosscut by lamprophyric and phonolitic dyke rocks.

Alkali feldspar, nepheline and sodalite are the dominant felsic minerals in all units. Plagioclase occurs sparsely. Clinopyroxene following the diopside-aegirine trend (Di₈₀Hd₁₂Ae₀₅ – Di₀₂Hd₀₉Ae₈₉) is the dominant mafic mineral together with biotite, locally occurring amphibole, magnetite and ilmenite. Titanite (up to 0.158 LREE³⁺ apfu) and fluorapatite (Fap₄₉Hap₄₇Clap₀₄ – Fap₁₀₀Hap₀Clap₀ with up to 0.664 LREE³⁺ apfu) occur as accessory phases throughout the complex, whereas fluorite and zircon occur as traces. Late magmatic phases include HFSE-bearing silicates of the wöhlerite- and rinkite-groups, documenting a transition from miaskitic to agpaitic phase assemblages during differentiation.

Alteration by hydrothermal fluids affected the whole complex. Aqueous fluids resulted in significant growth of zeolites, carbonate-bearing fluids formed calcite and cancrinite, mostly at the expense of nepheline. Breakdown of HFSE- and F-bearing silicates by carbonate-bearing fluids led to the growth of secondary fluorite, calcite and zircon, whereas breakdown of titanite caused formation of calcite and anatase.